What other suggestions for carpeting that would work better?
i'll let/hope someone else more experienced than i cuts in here, but from my standpoint i don't think any true groundcover plant will grow at a depth of 18 inches, under 96W and with no C02 (although you may be planning on adding this and just haven't mentioned it).
the only way I can see you having much luck with your current parameters and wishes for MM/HC groundcover is for some combination of the following:
top of the line substrate
growing tank emersed for a LOT of time (3, 6 or even 6+ months)
potentially upgrading your light and/or injection C02 once you were to fill your tank with water.
As for more readily-growable groundcover - you might want to consider some sort of moss. You could have a thick lush carpet of java moss or similar covering a lot of space within a month or so. If you were to go this route, you'd want to design a way to have it growing out of an anchored mesh screen.
Groundcover is a tough one - definitely do a search on here to see what other's have to say.
In my experiences, the two plants you're looking at are going to give you some difficulties:
- HC looks awesome but I can't imagine trying to grow a significant amount in a 46G.
- MM sends out runners so far away from each plantlet that it is tough to get it to act and true groundcover. It creeps along the ground more than spreading out over-top of it. Hard to explain but I can certainly send you some pictures if you like.